1. Field of the Invention
The present invention relates to a modulation method and apparatus applicable, for example, to an in-building modem used for communications within a building, or a wideband modem used for wide area communications, in the field of data communications.
2. Description of the Related Art
Traditionally, modulation for data communications is accomplished by implementing, within a digital signal processor (DSP), a signal point generator for generating signal points (symbols) representing transmission data, a low-pass filter (also called a waveshaping filter, roll-off filter, etc.) for performing interpolation while limiting signal frequency band, and a modulator for shifting the signal frequency band from baseband to passband using a carrier. In the signal processing, computations using complex vectors are carried out.
In recent years, the need for long-distance transmission using an in-building modem or a wideband modem has been growing. In long-distance transmission using an in-building modem or a wideband modem, it is required that the carrier frequency be set as low as possible to reduce signal attenuation and minimize energy loss at high frequencies. Furthermore, since the modulation rate increases with increasing modem speeds, the amount of computation must be reduced accordingly.
However, with prior known modulation methods, if it is attempted to simply lower the carrier frequency, the negative portion of the signal frequency spectrum is folded into the positive side at the instant that conversion is made from a vector signal (complex number) to a scalar signal (real number) for output onto a transmission line; in this case, there is the possibility that the folded portion may overlap into the baseband, causing noise and resulting in signal degradation. To subdue the noise caused by the overlapping (aliasing noise), the roll-off factor of the low-pass filter must be decreased, but if the roll off factor is decreased, the amount of computation in the DSP increases.